(a) Field of the Invention
The present invention relates to a method for mounting a semiconductor chip on a circuit board and, more particularly, to a method for mounting a semiconductor chip by using a flip-chip bonding technique.
Description of the Related Art
Flip-chip bonding technique is generally used for mounting a semiconductor chip on a circuit board. Patent Publication JP-A-2(1990)-285650 describes a flip-chip bonding technique, wherein retaining force by a thermosetting insulative resin is used for electrical and mechanical connection between a semiconductor chip and a circuit board. FIG. 1 is a flowchart for the flip-chip bonding process described in the publication, and FIG. 2 is a cross-sectional view of a semiconductor chip and a circuit board after the flip-chip bonding process of FIG. 1.
In step S301, an insulating resin is supplied onto a circuit board 11 after positioning electrodes 15 of the circuit board 11 to a specified location. In step S303, a semiconductor chip 13 having electrodes or bumps 14 thereon, which are formed beforehand in step S302, is positioned with respect to the circuit board 11, and then pressed against the circuit board 11 to contact the bumps 14 of the semiconductor chip 13 with the electrodes 15 of the circuit board 11, which are subjected to deformation by the pressing. After heating the insulating resin while pressing the semiconductor chip 13, the insulating resin 12 is cured to hold the semiconductor chip 13 and the circuit board 11 in a unitary body, thereby connecting the bumps 14 of the semiconductor chip 13 to the distorted electrodes 15 of the circuit board 11. In step S104, after-curing of the insulating resin 12 completes the mounting process for mounting the semiconductor chip 13 onto the circuit board 11.
In the flip-chip bonding process as described above, since the circuit board 11 is heated in the mounting step S303 up to about 200.degree. C. for curing the thermosetting insulative resin 12, volatile ingredients volatilizes from the surface of the circuit board 11 to generate voids 16 within the thermosetting insulative resin 12.
Especially in the case of a built-up circuit board generally used for a high density mounting, the built-up circuit board comprises a photo-sensitizing epoxy resin in the insulating layers thereof and, among others, cyclohexanone volatilizes from the epoxy resin to generate the voids 16.
Moreover, since the semiconductor chip is first pressed against the circuit board in step S303 without heating the insulator resin, the insulating resin 12 having a high viscosity tends to receive therein air from outside. After heating the insulating resin 12, in step S303, the received air expands to form large voids 16.
Those voids 16 cause to degrade the reliability of electrical and mechanical connection between the semiconductor chip 13 and the circuit board 11 irrespective of the materials of the ingredients of the voids 16.
Patent Publication JP-A-59(1984)-114891 proposes to solve the problem voids due to generation of gases from the circuit board in a soldering technique. In this publication, the circuit board is heated for a short time in a saturated steam of a liquid having a boiling point above the boiling point of the solder, to remove the volatile ingredients from the circuit board before the soldering and thereby avoid generation of blow holes in a through-hole during the soldering step. The blow holes are generally formed by the gases contained in the organic materials in the circuit board 11 and expanding due to the heat during the soldering to remain in the solder.
However, this process is only applied to a soldering process for 60 seconds at a temperature of 215.degree. C. and thus insufficient, even if applied to mounting a semiconductor chip on a circuit board, for removing the volatile ingredients from the circuit board.
Patent Publication JP-A-2(1990)-137393 proposes pre-bake of a circuit board in a vacuum ambient at a temperature between 50.degree. C. and 150.degree. C. However, this process is only effective to water content or moisture in the circuit board and not effective to the volatile ingredients.